Question

The atomic number of chromium is 24. Its electronic configuration in ground state is $1{s^2}\,2{s^2}\,2{p^6}\,3{s^2}\,3{p^6}\,4{s^1}\,3{d^5}$. Chromium atom by losing 3 electrons from ${\text{C}}{{\text{r}}^{3 + }}$ ions. A chromium atom contains $17\% $ more neutron than the protons. Now answer the following questions.
The number of unpaired electrons in ${\text{C}}{{\text{r}}^{3 + }}$ ions is:
A. 3
B. 6
C. 5
D. 1

Answer 1

Hint: The arrangement of electrons in the energy levels around the nucleus of an atom is known as the electronic configuration. The electrons fill up in the energy levels according to the Aufbau’s principle. We can determine the number of unpaired electrons of ${\text{C}}{{\text{r}}^{3 + }}$ ion from the electronic configuration.

Complete step by step solution:
We are given that the atomic number of chromium is 24.
The Aufbau’s principle states that in the ground state of the atoms, the orbitals are filled with electrons in order of the increasing energies. The order of energy of different orbitals in an atom is as follows:
$1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d$ and so on.
Thus, the electronic configuration of chromium is as follows:
$1{s^2}\,2{s^2}\,2{p^6}\,3{s^2}\,3{p^6}\,4{s^1}\,3{d^5}$
We know that the chromium atom by losing 3 electrons from ${\text{C}}{{\text{r}}^{3 + }}$ ions. The three electrons are lost from the valence orbitals of the chromium atom. One electron is lost from the 4s orbital and two electrons are lost from the 3d orbital.
Thus, the electronic configuration of ${\text{C}}{{\text{r}}^{3 + }}$ ion is as follows:
$1{s^2}\,2{s^2}\,2{p^6}\,3{s^2}\,3{p^6}\,4{s^0}\,3{d^3}$
The total capacity of the d-orbital is ten electrons. Thus, all the three electrons in the d-orbital are unpaired.
Thus, the number of unpaired electrons in ${\text{C}}{{\text{r}}^{3 + }}$ ions is 3.

Thus, the correct option is (A) 3.

Note:
Other rules that explain the electronic configuration are as follows:
1. Pauli’s exclusion principle: The Pauli’s exclusion principle states that the two electrons in an atom cannot have the same set of all four quantum numbers.
2. Hund’s rule of maximum multiplicity: The Hund’s rule of maximum multiplicity states that when several orbitals of equal energy are available, the electrons first fill all the orbitals singly before pairing in any of these orbitals.